Bought it a week ago and want to cook tacos with it tonight. Is that safe?
How does it look/smell? Anything "off" look or smell wise is a dead give away (pun intended). Never more than with meat -- "when in doubt, throw it out". Saving $X on meat by not replacing is never worth the illness/doctors' costs that might ensue. And, if budget is an issue, use canned black beans to replace the meat (cook up the same way you would the meat -- seasonings and all) -- you may discover a new preference! (I do this all the time -- replacing ground beef with black beans in Mexican food -- not only does my family not complain, most of the time they don't even notice. Healthier and cheaper -- how great is that??!)/
A week is a long time to keep ground beef even refrigerated.
My Snidely Whiplash scheme is to substitute SPAM in what I refer to as Tiki Tacos. That stuff never goes bad! And if you disguise it well enough people actually love it.
Ground beef's particularly susceptible to spoilage because of the surface area - firstly it comes into contact with the grinding surfaces and then is exposed to air & moisture as it sits. I'd toss.